Glass optical fibers require protective coatings to preserve the inherent strength of the glass and to buffer the fiber from microbending induced attenuation. Two coatings are generally used. The first coating, which is applied to the surface of the optical fiber, is generally referred to as the primary coating. The primary coating, once cured, is a soft, rubbery material that serves as a buffer to protect the fiber by relieving the stress created when the fiber is bent, cabled or spooled. The secondary or outer coating is applied over the primary coating. The secondary coating functions as a hard, protective layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the primary coating. The primary coating must maintain adequate adhesion to the glass fiber during thermal and hydrolytic aging, yet be strippable for splicing purposes.
The modulus of the primary coating must be low to cushion and protect the fiber by relieving stress on the fiber, which can induce microbending and, consequently, inefficient signal transmission. This cushioning effect must be maintained through a broad temperature range. Thus, it is necessary for the primary coating to have a low glass transition temperature (Tg). The low glass transition temperature will ensure that the coating remains in a rubbery state throughout a broad temperature range.
Another property the primary coating should possess is resistance to moisture. Moisture will rapidly degrade the strength of the primary coating and the underlying glass fiber during stress. Moisture will also adversely affect the adhesion of the primary coating to the glass, resulting in possible delamination. It is desirable for the coating to be as water resistant as possible. Moreover, the primary coating should be resistant to oils in filling compounds that are used to waterproof the fiber in the cable structure. Swelling of the primary coating by these oils can lead to signal attenuation.
It is also economical for manufacturers to apply the primary coating as rapidly as possible. Thus, the primary coating composition should undergo curing at higher line speeds.
Finally, the viscosity and shelf life of the primary coating is also important. Formulation stability of at least six to twelve months is considered good shelf life. Viscosity can typically be somewhat adjusted by regulation of the temperature at which the coatings are applied. However, it is advantageous to set the viscosity high enough so as to maintain proper rheology and handling of the coating, but low enough to facilitate bubble release and to minimize the amount of heat needed in the preparation. Excessive heat is undesirable because it may result in premature gelling or viscosity buildup due to possible thermal initiation of polymerization.
The use of urethane oligomers possessing terminal reactive groups in the primary coating for optical fibers is known in the art. U.S. Pat. Nos. 5,536,529 and 5,538,791 to Shustack; U.S. Pat. No. 5,336,563 to Coady et al; U.S. Pat. No. 5,373,578 to Parker et al.; and International Patent Application No. WO 91/03498 to Vandeberg et al. disclose the preparation and application of urethane oligomers to glass fibers and other articles. The preparation of the prior art urethane oligomer typically involves the reaction between an organic isocyanate compound and an alcohol. In some cases, the urethane oligomer possesses terminal acrylate and vinyl ether groups. None of these references, however, disclose curing the liquid polymers or oligomers of the present invention with functional monomers such as acrylates or vinyl ethers.
The use of urethane oligomers possessing terminal reactive groups in the secondary coating is disclosed in U.S. Pat. No. 5,015,709 to Birkle et al. and U.S. Pat. No. 4,902,727 to Aoki et al. These references, however, do not disclose curing a liquid polymer or oligomer of the present invention with functional monomers such as acrylates and vinyl ethers.
The use of epoxidized diene polymers possessing hydroxyl groups for the preparation of coating compositions is disclosed in U.S. Pat. No. 5,264,480 to Bening et al. These polymers react with isocyanates to produce coating compositions. There is no disclosure in Bening et al., however, of adding other components such as acrylates and vinyl ethers to produce the coating composition.
U.S. Pat. No. 5,536,772 to Dillman et al. discloses the preparation of a coating composition for an optical fiber composed of an unsaturated epoxidized diene polymer and a tackifying resin. The coating composition also contains radiation curable diluents such as acrylates, methacrylates, and vinyl ethers. There is no disclosure in Dillman et al. of the types of acrylates, methacrylates, and vinyl ethers that are used. Moreover, Dillman et al. does not disclose the use of two or more acrylate or vinyl ether monomers or an acrylate monomer in combination with another monomer to produce a primary coating composition.
U.S. Pat. No. 5,247,026 to Erickson et al. discloses the preparation of a coating composition composed of an epoxidized diene star polymer. Similar to the disclosure of Dillman et al., Erickson et al. also discloses that the coating composition also contains radiation curable diluents such as acrylates, methacrylates, and vinyl ethers. However, Erickson et al. does not disclose the use of two or more acrylate or vinyl ether monomers or an acrylate monomer in combination with another monomer to produce a primary coating composition.
European Patent Application No. 0 124 057 to Pasternack et al. discloses coating an optical fiber with an ultraviolet-initiated, cationically curable polyepoxide, a polysiloxane with a plurality of hydroxyl groups, and a photoinitiator and/or photosensitizer. There is no disclosure in Pasternack et al. to use functionalized monomers such as acrylates or vinyl ethers in the coating composition.
In light of the above, it would be very desirable to have a primary coating composition that possesses enhanced or increased thermal and hydrolytic stability. Another object of the present invention is to produce a primary coating that increases or enhances the static fatigue strength of a glass fiber. Finally, it would be advantageous to use starting materials that are less expensive than those used in the prior art to produce a primary coating composition for a glass fiber. The present invention solves such a need in the art while providing surprising advantages.